A critical building block in a radio receiver is the low noise amplifier (LNA). The LNA amplifies the received signal and boosts its power above the noise level produced by subsequent circuits. As such, the performance of the LNA greatly affects the sensitivity of the radio receiver.
FIG. 1 shows a diagram of a typical radio receiver 100. The radio receiver 100 includes a LNA 102 that receives radio frequency (RF) signals from an antenna 104 and amplifiers the RF signals for use by subsequent radio circuits. For example, the radio receiver 100 also includes a band pass filter (BPF) 106, a variable gain amplifier (VGA) 108, and a low pass filter (LPF) 110, which are used to demodulate the received signal.
The sensitivity of the radio receiver is a measure of the minimum signal that can be detected and demodulated. The minimum detectable signal (MDS) can be found using the expressionMDS=kTFTBwhere kT is thermal noise floor, FT is the noise factor of the radio receiver, and B is the bandwidth of the system. It follows then that the sensitivity of the radio receiver is the minimum detectable signal adjusted by the signal-to-noise ratio (SNR) required for proper demodulation, i.e.,Sensitivity=kTFT(SNR)minThe noise factor FT of a cascaded radio receiver is described by the well known Friis' equation as
      F    T    =            F      1        +                            F          2                -        1                    G        i              +          …      ⁢                                    F            2                    -          1                                      ∏                          i              =              1                                      n              -              1                                ⁢                                          ⁢                      G            i                              where Fi is the noise factor and Gi is the power gain of the ith stage. Note that the gain of the first few stages reduces the noise contributed by later stages. As a result, the first stage or first few stages set the noise factor of the radio receiver system.
The selectivity, or linearity, of the radio receiver is another important characteristic, and is a measure of the largest interfering signal that can be rejected by the system. Interfering signals can interact and produce intermodulation distortion products that degrade the quality (signal-to-distortion ratio) of the signal. The intermodulation distortion power increases rapidly with the interfering signal power. As a result, the later stages in a cascaded system have a profound effect on the overall linearity. This is because the gain of the earlier stages increases the amplitude of the signals applied to the later stages.
The sensitivity (cascaded noise factor) and selectivity (overall linearity) of the radio receiver create conflicting requirements. It is therefore advantageous to adapt the radio receiver to the operating environment.
In practice, the radio is switched between two operating modes—the first mode sets the LNA to its high gain value to receive low power radio signals, while the second mode by-passes the LNA to handle strong interfering signals. The LNA is switched from high gain mode to bypass mode when the received signal power is significantly above the minimum detectable signal level. Typically, this level is slightly larger than the gain difference between the two settings of the LNA.
FIG. 2 shows a graph 200 that illustrates the process of switching the LNA of the conventional radio receiver between the two operating modes. When the received signal power is low, the LNA is switched to its high gain value at shown at 202. When the received power is high, the LNA is switched to its low gain value as shown at 204.
Unfortunately, the switching approach described above places great stress on the linearity of the radio system just prior to switching the gain of the LNA, and amplifies the received signal power high above the receiver's noise floor—much more than required. Therefore, what is needed is a radio receiver that solves the above-described problems while providing excellent sensitivity and linearity.